Use the gcc visibility attributes when building with the MinGW compiler(gcc).
GCC also supports the __declspec syntax but it will not compile at the moment
until the issues(which may not even be exactly the same issues as with MSVC)
are resolved.
This avoids having to define define LIBFOO_DLL=1 all over the place. If we ever go with static libs we will
need to define LIBFOO_STATIC=1 but hopefully in some central location like the top level wscript.
Oh, and I also dropped support for gcc older than version 4.x because ardour will already not build
on such an old version.
